I'd say pick someone who can easily pass for someone who acts very approachable and charming in the public light, so that when he finally shows his true colors, or snaps if pushed to his limit, it comes as a shock. And even when he's done that, he always has to have that smug look on his face, like he's not afraid to let you know that he has something up his sleeve or knows something you don't, but won't tell you what it is.
The key to his character is that he thinks he's better than anyone else, knows he's playing his opponents, and enjoys every minute of it. He's a charming character, even when he's trying to take down or kill other characters, but you still can't help but love to hate him.
I've heard people suggest Matt Damon for the part, and I agree. He can play friendly and charming, but also smug, creepy and psychotic.
